Photoelectric device



July 14, 1936. w, HmK K 2,047,369

PHOTOELECTRIC DEVICE Filed D90. 21, 1934 L PM I INVENT =WILLARD HICK ATTORNEY.

Patented July 14, 1936 UNITED STATES PHOTOELECTRIC DEVICE Willard Hickok, Bloomfield, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 21, 1934, Serial No. 758,566

6 Claims.

My invention relates to electronic tubes and principally to tubes of the cathode ray type and their component parts. More particularly the invention is directed to the fabrication of screens adapted to serve as the photosensitive plaque in electron tubes which are to be used for the transformation of light energy into electrical energy as in television.

The cathode ray'tubes known in the prior art are usually so formed as to include an elongated envelope at one end of which there is positioned an electron emitting cathode of the direct or indirectly heated type. The electrons emitted from this cathode are focused and directed along the axis of the tube by suitable anode arrangements. The stream of electrons thus assumes the form of a beam often called the primary electron beam. This beam of electrons may be made to sweep out any predetermined area on the end of the tube opposite that containing the cathode by suitable arrangements and combinations of electrostatic deflecting plates or eletromagnetic deflecting coils to which appropriate wave formations of electric energy are applied.

The tubes of this type are usually constructed with one or more anode surfaces having impressed thereon suitable positive voltages, and in some tubes, especially those designed for certain systems of television transmitting and receiving, one of these anode surfaces may take the form of a fine wire mesh screen. The wires of the mesh are suitably insulated and the insulated surface is suitably treated and photosensitized to form a mosaic electrode of an infinite number of separate and distinct phototubes. A mosaic elec trode structure of this type is described by K. Tihanyi in copending application Serial No. 369,598.

Heretofore, in many cases, such mosaic screens have also been made by the formation of minute photosensitive globules or particles on one side of an insulating member, the reverse side of the insulating member being covered with a conducting material. The effect of this combination is to produce a great number of individually insulated condensers each capable of storing and retaining an electric charge. In operation, the light image to be transmitted is projected upon the surface of the mosaic screen, causing the individual photosensitive particles each to assume an electric charge-the intensity of the respective charges being dependent upon the intensity of light falling on the isolated photoelectric area. During the transmission of the image signals the charges acquired by the individual condensers having as one electrode of each the individual photosensitized areas are neutralized by the developed beam which is so controlled as to pass over or scan the mosaic surface according to the prearranged pattern. The effectiveness of this apparatus is dependent, to a great extent, upon the capacity of the individual condenser elements to retain a charge which will be proportional to the incident light thereon.

Now, in this type of mosaic electrode, various sections may conceivably be at different electric potentials. This condition gives rise to a circulation of secondary electron current which flows from the lesser charged particles to those particles more highly charged. The result of this circulation of current may cause considerable distortion and loss of detail when this type of mosaic is used in picture television systems.

A further difiiculty with the type of mosaic electrode indicated is that the system is often so arranged that the same side of the mosaic screen upon which the image is focused is scanned with the electron beam. This is, in turn, disadvantageous in that the mosaic electrode must be disposed at an angle to the electron gun or other electron source so that if distortion in the reproduced electro-optical image is to be avoided special electron beam scanning means are required or provision must be made for compensation for the so-called keystone effect. One form of keystone compensation system is disclosed in British Patent No. 412,092 although other forms may be used.

To overcome certain of the foregoing diiflculties it has been proposed to use a double-sided mosaic. A double-sided mosaic electrode may be broadly described as one which is made on a foundation of a suitable insulating base, so constructed that it may be scanned on one side by the electron beam while the light image of the picture is made incident upon the opposite side. The primary electron beam is thereby separated from the photoelectric emission. This separation is advantageous in that the secondary electrons, released at the point of impact of the primary electron beam with the anode or mosaic surfaces, are separated from the electrons released from the photosensitive surface. The two groups of electrons from the two sources will thus be prevented from intermixing if the screen or mosaic electrode is constructed as described in what follows.

Among the objects of my invention are: first, to provide a cathode ray television transmitting tube having high sensitivity; second, to provide a cathode ray television transmitting tube in which it is possible to combine high sensitivity with good picture resolution; third, to provide a cathode ray television transmitting tube in which the insulation of the mosaic surface is considerably strengthened; and fourth, to provide a simplified and more uniform method of manufacture for mosaic screens for use in cathode ray television transmitting tubes.

My invention possesses numerous other objects and features which will be set forth in the following description of my invention. It is to be understood that I do not intend to limit myself to the particular modification shown as I may adopt variant embodiments of my invention within the scope of the appended claims.

Broadly considered, my invention comprises constructing a cathode ray tube having improved characteristics in that it incorporates an improved mosaic anode structure having a wire mesh foundation. The wires are interwoven in the form of a screen having interstices of the order of ten thousand to one hundred thousand per square inch. In screening of this size the ratio of open spaced area to-total area is relatively small in that it is impractical to weave the screen of exceptionally fine wire. In addition, it is necessary to provide a surface on which a uniform coat of enamel may be applied so that the subsequent inlaying of the interstices with metal will produce a plurality of individual insulated inserts equal in number to the interstices of the screen. This requirement makes necessary an operation of rolling the screen to imbed the wires of the woof with the wires of the weft. This operation necessarily decreases to a considerable extent the ratio of open spaced area to the total area. In accordance with my invention such screens are rendered more effective by considerably increasing this ratio of open spaced area to total area. This effect is conveniently brought about by means of a suitable etching process to be herein described.

The novel features which I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, will best be understood from the following description of a specific embodiment when read in connection with the accompanying drawing, in which Figure 1 is a view partly in elevation and partly in axial section of an electron discharge device embodying my invention;

Figure 2 is a view of a mosaic screen made in accordance with my invention;

Figure 3 is a greatly enlarged perspective view of a section of mosaic screen made in accordance with my invention;

Figure 4 represents a sectional view of Figure 3 on the line 4-4 looking in the direction of the arrows; and,

Figure 5 is a greatly magnified view of a section of the mosaic screen structure after the preparation thereby is completed prior to photosensitizing the same.

In detail the apparatus shown in the drawing Figure 1 comprises an evacuated vessel [0 which usually takes the form of a neck section I! and a cylindrical section i4 terminated by a substantially plane surface IS. The re-entrant stem is used to support the electron gun assembly l8 comprising the filament or heater 20, the cathode 22, a control grid or disc 24, and first anode 26. By virtue of the electrical potentials applied from an appropriate source (not shown) to the com-'- ponent electron gun parts, a beam of electrons, conventionally shown at 28,'may be drawn from the cathode and directed along the longitudinal axis of the tube. Suitable deflecting elements 30 and 32 to which proper scanning potentials may be applied are provided for the purpose of sweeping the beam across and along the mosaic screen 34 at the end of the tube opposite the electron source.

The deflecting elements may take the form shown; that is, combined electrostatic-electromagnetic means, or electrostatic plates may be used for both coordinate deflections or, if desired, two sets of electromagnetic coils may be used. A second anode 36 is formed upon the interior of the retaining walls of the frustoconical tube section l4 for the purpose of further I accelerating and focusing the beam 28 as it moves from the first anode 26 to the mosaic electrode 34. This second anode 36 also serves the purpose of collecting the secondary electrons emitted from the surface of the mosaic electrode nearest the electron source 22 by reason of the high velocity beam of electrons being incident upon the screen.

The mosaic electrode 34 has for its foundation a wire mesh screen composed of finely woven wires 35, as shown more particularly by Figures 3 and 5. Upon the surface of the mesh of wires 35 is an insulating material 38 (see for example Figure 5) and a filling of metallic silver 40 or other oxidizable metal is pressed into the interstices 42. The screen assembly 34 is then mounted in the tube by suitable support leads 44 and surface 46 thereof is photo-sensitized by a process such'as used in alkali photo-tube manufacture. An auxiliary anode 48 is provided to collect the electrons emitted by the photo-sensitive surface 46.

The mosaic electrode structure 34, above referred to, has as a foundation a closely woven wire mesh screen. It has been found that for good detail in the reproduced electro-optical image it is necessary to provide mosaic screens having two to three hundred or more wires per linear inch, if the mosaic is to be held to practical dimensions.

In accordance with my invention the mesh screen, following the weaving process, is etched in a suitable acid solution. Any mesh or screen of metal which is uniformly corrosive to the action of acids or other reagents may be used for the foundation of the mosaic. It has been found, however, that the enamels which are used as insulating coatings are more adherent to some surfaces than to others. Nickel, Monel metal, and stainless steel are some metals which have been found to be very satisfactory for this purpose. By the use of suitable enamels, however, there is no reason to be limited to these specifically mentioned. steel wire, the etching solution may consist of 35% to 40% aqua regia in water; the solution being at 30 C. This concentration gives a relatively slow reaction which results in a very uniform etching of the metal. The speed of reaction of the solution may be made more uniform by adding thereto, before the process of etching, a small quantity of iron dust or other corrosive metal.

An understanding of the action may be had by the following example: a 200 wire per linear inch plain weave mesh was woven using wire of .0022 inch in diameter. After rolling the wire cloth, the opening size was .0015 inch and the solid part .0035 inch. This was equivalent to a reduction in opening area of over 50%. The mesh was immersed in the aqua regia solution for a period of 30 seconds, whereupon the wire size was reduced to .0014 inch resulting in an opening area of .0036 inch. The open area was thus increased by 240%.

For nickel similar results may be obtained by etching for approximately 45 seconds in a 60% nitric acid solution in water at 30 C. Solutions If the screen is of stainless and constants for other metals may be determined by those skilled in the art it it is understood that one of the prime requisites of the process is to etch the metallic surface slowly so that a uniform surface may result. Following the etching process, and providing an acid is used in said etching, the wire mesh is immediately cleaned in an alkaline bath to remove any acid.

As may be seen from the foregoing example, the wire was etched to such an extent as to be reduced by approximately 60% in area. This entails considerable reduction in strength of the finished mosaic and it is, therefore, advisable, in accordance with my invention, to electroplate the screen after the cleaning process. Silver and nickel have been found to be very satisfactory for this use, a deposit of either metal being made just sufficiently heavy to bond securely the wires of the mesh. It is desirable to use as thin a deposit as possible to prevent filling of the interstices with excess metal.

As has been previously indicated, the mesh screen is then coated with a layer of suitable insulating material. An enamel comprising feldspar, borax, and silica in combination with a suitable bonding agent, such as cobalt oxide, may be used. This enamel may be applied most conveniently by spraying the screen therewith. It has been found that dipping the screen in the enamel is disadvantageous in that the interstices are apt to be filled completely with the enamel. screen is baked at a relatively high temperature for the purpose of vitrifying the enamel.

Subsequent to this baking of the enamel, a silver or other metallic paste is packed or pressed into the screen structure in such manner that the interstices are completely filled with the material. The silver or metallic paste may consist of finely divided metal with a binder or an easily reducible oxide of the metal with a binder. Just sufficient material is applied to fill the interstices forming in efiect a great number of individually insulated plugs, but the metal should not be permitted to extend beyond and over the insulated wires, for if it should, the individually isolated conducting areas would not result. The screen is then heated to reduce the oxide or vaporize the binder, de

pending on the material used, whereupon the screen 34 may be sealed into a tube I0, as shown by Figure 1.

When so positioned, the screen 34 is intermediate the conducting secondary anode surface 36 and the tertiary anode surface 48.

During the process of evacuation the prepared mosaic electrode 20 is photosensitized by depositing caesium or other photosensitive material on the surface thereof by any suitable and desired method. One which has proven satisfactory is that disclosed by Sanford F. Essig in his copending application, Serial No. 549,779, filed February 24, 1932.

In the operation of the arrangement hereinabove described the light image of an object is focused by means of a suitable optical system, such as the lens 52, upon the mosaic screen 34, as in Figure l, whereupon various positive charges are formed on the surface of the said mosaic because of the emission of electrons to the positive anode ring 48. The beam of electrons 28 from the cathode 22 is focused and directed by the focusing gun 26 and suitably deflected by the deflection means 30--32 upon which is impressed electrical energy of suitable wave formation to deflect the beam along the desired paths.

Following the spraying process the The electron beam 28 in scanning the mosaic electrode 34 neutralizes the positive charges induced upon the various sections of photosensitive material 46 on the image side of the mosaic electrode. The electrons from the beam return to the positive anode 36 which is maintained at a potential relative to the cathode slightly less positive than the anode ring 48. The mosaic electrode 34, together with the anode 36, may form an output circuit, such as that described in U. S.

limited to the exact construction shown or limited as to the materials and percentages thereby used, although the examples illustrated have proved satisfactory, and therefore many and various modifications may be resorted to without departing from the spirit and essential attributes of the invention as defined in the appended claims.

What I claim and desire to secure by Letters- Patent is the following:

1. The process of fabricating mosaic electrodes from a wire mesh screen which comprises etching the screen structure in a corrosive bath to increase the area of the interstitial spaces, coating the screen structure with an electrically insulating substance, and filling the interstitial spaces with an electrically conductive material.

2. The process claimed in the preceding claim comprising in addition the step of photosensitizing one side of the electrically conductive material supported within the interstitial spaces of the mesh.

3. The process of fabricating mosaic electrodes from a wire mesh screen which comprises etching the screen structure in a corrosive bath to increase the area of the interstitial spaces, rolling the said wire mesh screen to imbed the wires of the warp with the wires of the weft at their intersections, coating the screen structure with an electrically insulating substance and filling the interstitial spaces with an electrically conductive material.

4. The process claimed in the preceding claim comprising in addition a step of photosensitizing one side of the electrically conductive material supported within the interstitial spaces of the mesh.

5. The process of fabricating mosaic electrodes from a wire mesh screen which comprises etching the screen structure in a corrosive bath to increase the area of the interstitial spaces between the warp and the weft of the wire mesh, rolling the said wire mesh screen to imbed the wires of the warp with the wires of the weft at their intersections, subsequently electroplating a metallic material upon the wire mesh to bind all wires of the mesh closely together at their points of intersection, coating the wire mesh structure with an electrically insulating substance, and filling the interstitial spaces of the mesh with an electrically conductive material.

6. The process claimed in the preceding claim which comprises in addition the step of photosensitizing one surface of the electrically conductive material within the interstitial spaces of the mesh.

WILLARD I-HCKOK. 

